7-TM Receptors7-Transmembrane receptors (7-TM receptors), also known as G-protein-coupled receptors, or GPCRs, are integral membrane proteins that contain seven membrane-spanning helices. As the name suggests they are coupled to heterotrimeric G proteins on the intracellular side of the membrane. Upon ligand binding, the GPCR undergoes a conformational change which is transmitted to the G protein causing activation. Further signal transduction depends on the type of G protein.Classification7-TM receptors are activated by a wide variety of ligands including light, olfactory stimulants, peptides, hormones and neurotransmitters. They are grouped into 6 classes based on sequence homology and functional similarity: Class A (or 1) (Rhodopsin-like) Class B (or 2) (Secretin receptor family) Class C (or 3) (Metabotropic glutamate/pheromone) Class D (or 4) (Fungal mating pheromone receptors) Class E (or 5) (Cyclic AMP receptors) Class F (or 6) (Frizzled/Smoothened)Physiology and DiseaseThe human genome encodes roughly 350 7-TM receptors, of which approximately 150 of these have an unknown function. They comprise the largest family of receptors in the human genome and because of their large number, widespread distribution and important roles in cell physiology and biochemistry, they play an important role in medicine.Diseases involving mutations of 7-TM receptors are relatively rare, however disorders which involve antibodies directed against 7-TM receptors are more common. Typically disease can be caused by: Non-functional receptors (GHRH and familial growth hormone deficiency) Constitutive activation (Rhodopsin and retinitis pigmentosa) Changes in ligand binding specificity (Thyroid-stimulating hormone receptor and hyperthyroidism of pregnancy) Improper receptor processing (vasopressin receptor and diabetes insipidus) Antibodies directed against the receptors (Thyroid stimulation hormone receptor and Graves disease) Constitutively active or inactive G proteins7-TM receptors are the target of around half of all modern medicinal drugs. Their expression on the cell surface makes them readily accessible to hydrophilic drugs and their non-uniform expression provides selectivity in activating or blocking physiological events. Agonists and antagonists of 7-TM receptors are used in the treatment of disease in every organ system.Muscarinic receptorsare widely distributed throughout the body and control distinct functions according to location and subtype (M1- M5).They are predominantly expressed in the parasympathetic nervous system where they exert both inhibitory and excitatory effects.Receptor SubtypeM1M2M3M4M5

Transduction MechanismPLC (Gq/11)Adenylyl Cyclase (Gi/o)PLC (Gq/11)Adenylyl Cyclase (Gi/o)PLC (Gq/11)

Primary DistributionNeocortex, hippocampus, neostriatum, salivary glands, eye, heartBrain, heart, smooth muscle, salivary glandsSmooth muscle, exocrine glands, brain, autonomic ganglia, salivary glandsNeostriatum, cortex, hippocampus, autonomic gangliaSubstantia nigra, pars compacta, ventral tegmental area, hippocampus

Tissue FunctionLearning and memory, neuromodulation, M-current inhibitionBradycardia, smooth muscle contraction, antinociception, hyperthermia, neurotransmitter release (presynaptic)Smooth muscle contraction, gland secretion, neurotransmitter release (presynaptic)Regulation of dopamine D1stimulated locomotor activityUnknown